Making a submission is as easy as writing a letter saying "I support/oppose this bill" and giving reasons. The Office of the Clerk has a helpful guide here, and my draft submission on this bill (which I'll be reworking over the next few days) is here. Opponents of the bill may also want to pay attention to my post summarising the Royal Commission on the Electoral System's arguments on the size of Parliament here.

Update: Fixed URL and corrected number of copies. You only need to send two now rather than twenty.